That's exactly what's going on with ReCAPTCHA. One of the elements presented is a "known" that you have to get right, and the other is (usually) an unknown or a

That's exactly what's going on with ReCAPTCHA. One of the elements presented is a "known" that you have to get right, and the other is (usually) an unknown or ambiguous OCR result. Google is crowdsourcing their scans. And yes, sometimes it is just a blob of ink -- on other occasions, it's an inline differential equation or something similar that can't be properly entered into the text box (even with the whole of Unicode at your disposal). There's no penalty for getting the graecum est part wrong, but if it's something you can read but can't enter properly, do be a gem and don't pollute the pool of results -- hit the refresh icon.